This invention relates generally to the field of high speed rotating machinery and more particularly to crushing machines of the type in which a stream of material such as ore, limestone, coal, rock or the like is fed to an impeller wheel or disk which accelerates the material to a high velocity and hurls it centrifugally against an adjacent target or impact surface where the material is broken up by the force of the impact.
A prior art crusher of this type is shown in U.S. Pat. Nos. 3,162,382 and 3,180,582 granted to Ostap Danyluke and assigned to the assignee of the present application. In the patented devices, the material particles leaving the impeller are caused to collect in an annular space outboard of the impeller, where they are subject to continued bombardment by additional particles thrown out by the impeller, producing additional crushing of the particles. While this type of autogenous crushing apparatus has some advantages such as minimization of wear on the impaction surfaces due to the protection afforded by the collected layer of material particles, centrifugal crushing systems similar to those disclosed by Danyluke have been subject to some vexing problems.
Factors such as shaft instabilities and self-induced vibrations at high speeds have resulted in excessive lateral vibrations and whirl in the impeller shafting. This vibration is aggravated by imbalanced conditions in the impeller wheels of prior art devices due, for example, to uneven impeller wear in operation, a clogged flow passage, manufacturing variations, or the presence of a single heavy particle in the impeller. Such lateral vibration is transmitted to the shaft bearings of both the impeller and the moving impact surfaces, with resultant high incidence of bearing wear and failure.
The lateral vibration of the drive shafting in such prior art crushing machines is thought to be a combination of two different types of vibration or movement: one, a forced or resonant type; and the other, a self-excited or instability type. In the resonant type, the most common driving frequency is the shaft speed or some multiple thereof. Some stimuli which have been noted for resonant vibrations are: rotor unbalance, in which the vibration is excited by the centrifugal force acting on the rotor's eccentric center of gravity; shaft misalignment, in which the rotor centerline is not true to the centerline of the bearings at either end of the shaft; and periodic loading applied to the rotor by external forces such as those mentioned previously.
Self-excited vibrations or instabilities are characterized by the presence of some sort of a mechanism which causes the shaft to whirl at or near its own natural frequency, usually independent of the frequency of shaft rotation and other external stimuli. Such self-excited vibrations are rather subtle and difficult to diagnose, .[.buth.]. .Iadd.but .Iaddend.are potentially quite destructive since whirling due to self-induced vibration induces alternating stresses in the shaft and rotor which can lead to fatigue failures.
These instabilities or self-excited vibrations, generally referred to as whirling or whipping, are characterized by the generation of a tangential force normal to the radial deflection of the rotating shaft. The magnitude of the force is proportional to, or varies monotonically with, the radial deflection of the rotating shaft. For a more complete discussion of such behavior, see "Identification and Avoidance of Instabilities and Self-Excited Vibrations in Rotating Machinery" by F. F. Ehrich, ASME publication No. 72-DE-21, Design Engineering Division, 1972 (10 pages).
In crushing machines of the type now under discussion, the self-excited tangential forces may be large enough to overcome the external damping forces of the device at some onset speed and thus induce a whirling motion of ever increasing amplitude, subject only to nonlinearities which ultimately limit deflections. Various instabilities such as hysteretic whirl, dry friction whip and fluid bearing whip are thought to contribute to this whirling phenomenon. The following discussion is not a complete catalogue of the mechanisms which contribute to whirl due to instability and self-excited vibration, but is presented only to illustrate the types of problems overcome or controlled by this invention.
In hysteretic whirl, a nominal shaft deflection induces a neutral strain axis normal to the deflection axis. Assuming the neutral stress axis is coincident with the neutral strain axis, the net elastic restoring force should be parallel to and opposing the deflection. However, hysteresis or internal friction in the shaft causes a phase shift in the development of stress as the shaft fibers rotate around through peak strain to the neutral strain axis. The result is that the neutral stress axis is displaced from the neutral strain axis so that the net elastic restoring force is not parallel to and opposing the deflection. The restoring force thus has a tangential component normal to the deflection which may be large enough to induce a whirling motion in the direction of shaft rotation. The whirling motion increases the centrifugal force on the deflected rotor, thereby increasing its deflection, thereby increasing the magnitude of the tangential component and so forth. Hysteretic whirl usually occurs only at speeds above the first critical speed of the shaft.
In dry friction whip, the surface of the rotating shaft comes in contact with an unlubricated stationary, or relatively slow moving, guide or shroud. When radial contact is made, friction will induce a tangential force on the rotor. Since the friction force is approximately proportional to the radial component of the contact force, instability can occur, as previously described for hysteretic whirl. In this instance, however, the whirl will be opposite to the direction of shaft rotation.
In fluid bearing whip, the shaft rotates in a gas or liquid filled clearance. The entrained, viscous fluid will circulate with an average velocity of about half the surface speed of the shaft. For a nominal radial deflection of the shaft, the bearing pressures will not be symmetric about the radial deflection line. Because of viscous losses of the bearing fluid passing through the close clearance, the pressure on the upstream side of the close clearance will be higher than on the downstream side. A tangential force results which tends to whirl the shaft in the direction of shaft rotation. When this tangential force is greater than the internal damping of the system, a whirl is induced, as previously described.
Other factors known to contribute to self-excited instabilities and their resultant whirl are asymmetric shafting and pulsating torque application which may cause parametric excitation. In the case of asymmetric shafting, sufficient levels of asymmetry in the flexibility associated with the two .[.principle.]. .Iadd.principal .Iaddend.axes of flexure of the shaft or rotor will cause periodic changes in shaft flexibility as the shaft rotates. This will cause instability at some speeds. The application of pulsating torque to a shaft affects its natural frequency in lateral vibration, which can cause instabilities in some speed ranges.
In addition to the above types of problems, the autogenous grinding machines of the prior art frequently require the use of higher power inputs or large numbers of repeated crushing cycles to produce desired size reductions, possibly due to dynamic inefficiencies of the impacts experienced between material leaving the impeller and the material collected on the impactor surfaces. Moreover, autogenous grinding tends to produce a large proportion of fines in the crushed product, which may be wasted in many applications where larger particles are desired.